A feedback amplifier used as a preamplifier of an optical communication system has an amplifier circuit to amplify an input signal. A feedback circuit is provided to control the level of an output voltage in the feedback amplifier and the feedback circuit includes a feedback resistor connected between an input terminal and an output terminal, and a feedback transistor connected to the feedback resistor in parallel. Herein, the feedback transistor receives a control signal through the base thereof to control the feedback current flowing through the feedback circuit, and control an output voltage.
There are two methods of applying the control signal to the base of the feedback transistor. That is, two methods include a method of generating the control signal by configuring an additional gain control signal generating circuit outside the feedback amplifier, and a method of generating the control signal by embedding a bias circuit inside the feedback amplifier.
FIG. 1 illustrates a circuit diagram of a feedback amplifier in the related art.
Referring to FIG. 1, a feedback amplifier 100 in the related art is connected between an input terminal IN and an output terminal OUT. Input terminal IN is connected to a photodiode PD connected to a power supply VCC. An input current corresponding to a cathode current of photodiode PD is supplied to input terminal IN.
Input terminal IN is connected to the base of a first NPN transistor 110. The collector of first NPN transistor 110 is connected to power supply VCC through a first resistor 120 and the base of a second NPN transistor 130. The collector of second NPN transistor 130 is connected to power supply VCC, and the emitter of second NPN transistor 130 is connected to output terminal OUT and to the ground terminal through a second resistor 140. Moreover, an emitter of first NPN transistor 110 is also grounded.
A feedback circuit 150 includes a feedback resistor 152 and a feedback transistor 154. Output terminal OUT is connected to the base of first NPN transistor 110 through feedback resistor 152. The emitter of feedback transistor 154 is connected to output terminal OUT, and the collector of feedback transistor 154 is connected to the base of first NPN transistor 110. A control signal V_AGC for controlling a feedback current is applied to the base of feedback transistor 154 from an external circuit (not shown).
In such feedback amplifier 100, in the case where control signal V_AGC having a low voltage is applied to the base of feedback transistor 154, feedback transistor 154 is in an OFF state while the impedance between the collector and the emitter of feedback transistor 154 fully increases. Therefore, it is considered that feedback transistor 154 is equivalent to an opened state and the transimpedance is equivalent to the resistance value of feedback resistor 152.
Thereafter, when control signal V_AGC increases, feedback transistor 154 is turned ON and therefore the impedance between the collector and the emitter of feedback transistor 154 is decreased. As a result, feedback resistor 152 and the impedance are connected to each other in parallel to decrease the transimpedance. Accordingly, the gain of feedback amplifier 100 is controlled in response to control signal V_AGC.
That is, in feedback amplifier 100, in the case where control signal V_AGC is changed in response to the input current, control signal V_AGC is determined based on the input current within a range in which an output voltage S100 is not saturated. In order to supply control signal V_AGC with a proper signal value, an additional gain control signal generating circuit should be configured, thereby supplying control signal V_AGC to feedback amplifier 100.
However, since the feedback amplifier in the related art merely controls the dynamic range by a turn-ON resistance of the feedback transistor, the feedback amplifier is limited in providing a wide dynamic range.